A highly stable full-polymer electrochemical deionization system: dopant engineering & mechanism study

Electrochemical deionization (ECDI) has emerged as a promising technology for water treatment, with faradaic ECDI systems garnering significant attention due to their enhanced performance potential. This study focuses on the development of a highly stable and efficient, full-polymer (polypyrrole, PPy) ECDI system based on two key strategies. Firstly, dopant engineering, involving the design of dopants with a high charge/molecular weight (MW) ratio and structural complexity, facilitating their effective integration into the polymer backbone. This ensures sustained contribution of strong negative charges, enhancing system performance, while the bulky dopant structure promotes stability during extended operation cycles. Secondly, operating the system with well-balanced charges between deionization and concentration processes significantly reduces irreversible reactions on the polymer, thereby mitigating dopant leakage. Implementing these strategies, the PPy(PSS)//PPy(ClO4) (PSS: polystyrene sulfonate) system achieves a high salt removal capacity (SRC) of 48 mg g−1, an ultra-low energy consumption (EC) of 0.167 kW h kgNaCl−1, and remarkable stability, with 96% SRC retention after 104 cycles of operation. Additionally, this study provides a detailed degradation mechanism based on pre- and post-cycling analyses, offering valuable insights for the construction of highly stable ECDI systems with superior performance in water treatment applications.


Section 1. Procedure for preparing titanium current collectors:
The titanium sheets, from Raysen Titanium Industry Co. (Taiwan), with geometric size of 7 cm  1 cm were immersed in a beaker containing 6 M HCl.The beaker was heated to 80°C and kept at this temperature for 30 min.Afterward, the pickled sheets were thoroughly rinsed with deionized water to remove any surface grease, followed by ultrasonic agitation for comprehensive cleaning.The titanium sheets were immersed in the RuO 2 solution (0.3 mg/mL in DI water) and dried in an oven at 80°C for three repetitions.The pretreated titanium sheets were annealed at 250°C for 2.5 h, resulting in a thin layer of RuO 2 forming on the substrate.The RuO 2 layer simultaneously acts as a protective layer against substrate oxidation and improves the coating uniformity of PPy, which provides no ion removal capacity.    Figure S5 shows the typical XRD patterns of various PPy films on the titanium substrates after synthesis.The black line indicates the XRD pattern of a titanium substrate, which most of the peak scatters between 35° to 80°.The responses of the PPy film can be mainly observed between 10° to 30° (2).However, due to the poor crystalline structure of PPy, the peaks display in a wide shape.The specific capacitance (C P ) was calculated through the following equation:

S7
(S1) where A is the area of the CV curve within a certain potential window, m indicates the mass of the electro-active material, k represents the scan rate, and is the potential ∆ window, which is 2 V (-1.2 V~0.8 V) in this case.

Figure S1 .
Figure S1.The setup of the desalination test.

Figure S7 .
Figure S7.Conductivity-time profiles of the 103 rd and 104 th cycles for

Figure S9 .
Figure S9.The sulfate concentrations in the 10 mM NaCl solution which was subjected

Table S2 .
Detailed values in Figure 4.
*After removing the bubbles in the system.+ Average energy consumption was calculated with the following equation:

Table S3 .
Specific capacitance (C P ) in Figure.S6 measured in 10 mM NaCl with a potential window of -1.2 V~0.8 V (vs.Ag/AgCl) at a scan rate of 2 mVs -1

Table S4 .
Comparisons of SRC, EC, membrane, cycle life, and retention between this work and various recently proposed conducting polymer-based or conducting polymer-